1. Field of Invention
The invention relates generally to air handling systems, and, more particularly, to compositions and methods of treating air handling systems to prevent microbial buildup and fouling of cooling coils.
2. Description of Related Art
Indoor air quality is a subject of great concern at present, since problems therewith are known to cause illnesses in the building's occupants, a condition known as "sick building syndrome." The primary source for this condition typically lies in the heating, ventilating, and air conditioning (HVAC) system, which can introduce particulates and microorganisms into the air the occupants are forced to breathe. This problem is compounded by most modern buildings being sealed from outdoor air except through the HVAC system, since windows are typically not openable.
The air that is pulled into a building's air handling system is filled with particulates such as dirt, mold spores, microorganisms, and pollen; most of these measure below 10 micrometers, which can easily pass through the filters commonly used at the HVAC system inlet.
Incoming air next usually proceeds through a cooling coils, which normally has 14-18 fins per inch, with approximately 1/16" between adjacent fins. During operation, these fins have a film of condensed water on their surfaces. When the particulate-laden air impinges on the coil, the condensate collects the debris and washes a portion of it into a drip pan, with the remainder adhering to the coils, on which mold and other microorganism growth may occur.
As a matter of practice, coils are cleaned with an acid or caustic cleaner. Since aluminum or copper, the materials of construction of most coil fins, are amphoteric metals, they will corrode in the presence of such cleaners. Such corrosion causes microscopic pitting on coil surfaces, which later harbor and are repositories for contamination such as dirt, microbial growth, and other debris.
The high velocity of air passing across the cooling coil then causes the debris to form an aerosol that is carried by the air stream, is breathed by the building's occupants, and contaminates duct work.
Several attempts have been made in the past to overcome the problems associated with airborne particulates circulated by HVAC systems. Mostly these attempts relate to sprays, coatings, or some related liquid being sprayed or otherwise applied to interiors of the HVAC system. There materials generally contain volatile organic compounds (VOCs). In recent years, increasingly strict legislation has been enacted restricting VOC use.
Smith et al. (U.S. Pat. No. 4,780,333) teaches the use of a deodorizer and antimicrobial coating to treat an air conditioning system. Szappannyos and Eckerman (U.S. Pat. No. 3,591,328) discloses a method of treating air conditioning coils with a fungicide by spraying into an air stream while the air conditioner is operating. Demuth (U.S. Pat. No. 3,493,323) teaches a process of spraying a sterilizing agent through a nozzle into HVAC ducts and then spraying them with a hot fluid, which must subsequently be drained.
Wachman et al. (U.S. Pat. No. 5,124,359) discloses a biocidal aqueous composition for spraying on nonabsorbent surfaces for use on dental and medical equipment. Similarly, Oakes et al. (U.S. Pat. No. 5,061,485) teaches another germicidal composition for disinfecting surfaces. McCoy et al. (U.S. Pat. No. 5,122,301) discloses various antimicrobial compositions but does not include a method of application.
Hayashi et al. (U.S. Pat. No. 4,783,006) describes a mist-dispensing device for forming a thin film on a substrate.
Terry et al. (U.S. Pat. No. 4,957,948) teaches a biocidal protective coating and method for coating heat exchange coils. The coating comprises a polymeric composition comprising an organic water-resistant polymer that bonds to the surface of the coils and a substituted phosphoric acid compound. The method for coating is broadly disclosed as "dipping, brushing or spraying the coils."
There are many problems with currently used compositions and their methods of applications: (1) They usually contain a harmful biocide that can be toxic during application; (2) they may contain volatile organic compounds, which are hazardous to the person applying them, negatively impact indoor air quality for the building occupants, and are restricted by law; (3) their surface coverage may reduce the heat transfer characteristics of the coils, which may cause compressor burnout and increase operating costs; (4) most are not designed to reduce coil fouling; and (5) the depth to which the composition penetrates during application is insufficient for coils over 2 inches deep.
It is thus an object of this invention to provide a composition and method for treating cooling coil surfaces that is capable of minimizing microorganism growth.
It is another object to provide a composition and method that will retain its biocidal characteristics for up to a year.
It is a further object to provide a composition that contains neither a highly toxic biocide nor any volatile organic compounds and is thus not harmful to the building occupants or the applier.
Another object is to provide a composition and method that does not form a film on metal surfaces and thus does not substantially alter the heat transfer characteristics of heat exchange surfaces. Instead, the composition clings to the coils via ionic interaction.
Yet a further object is to provide a composition whose application will prevent debris from adhering to heat exchange surfaces and thus minimize fouling.
It is another object to provide a method of applying a protective composition that ensures the coverage of even the deepest coils within the heat exchanger.